(1) Field of the Invention
The present invention relates to structures for preventing fall of screws, and in particular to a structure for preventing fall of a screw that is used to secure a removable unit to a main body of an apparatus.
(2) Description of the Related Art
For example, image forming apparatuses, such as copiers and printers, for forming images by electrophotography have a fusing device for thermally fusing toner images onto a recording sheet. For easy maintenance, it is typical to unitize the fusing device as a unit that is removable from the main body of the apparatus.
Such a fusing unit is secured to the apparatus by fastening a frame of the fusing unit (hereinafter, simply “unit frame”) to a frame of the apparatus (hereinafter, simply “main-body frame”) using screws. The main-body frame has a tapped hole in a mounting surface for the unit, whereas the unit frame has a through hole (hereinafter “insertion hole”) that is larger in diameter than the outer diameter (nominal diameter) of the screw. By passing the screw through the insertion hole and then fastening the screw into the tapped hole to make thread engagement, the unit frame is firmly sandwiched between the screw head and the mounting surface. As a result, the fusing unit is secured to the apparatus.
To remove the fusing unit from the apparatus, first, the screw is loosened to disengage the screw from the main-body frame. At this time, it is required to hold the screw with fingers, for example, in order to prevent the screw from falling out of the insertion hole. However, the space available around the part subject to the fastening is often very limited. Therefore, it may be difficult to reliably hold the screw with fingers. Furthermore, holding the screw with fingers may not be possible at all, in the case, for example, where the fusing unit was in use until a moment before the removal and thus the screw is hot.
In view of the above, a fall preventing washer has conventionally been used to prevent the screw from falling. Typically, a fall preventing washer is an annular plate having slits extending radially outwardly from the inner edge defining a hole. Thus, a tongue-like portion is formed between each adjacent slits which is flexible to allow for resilient flexure in the direction perpendicular to the diametrical direction of the hole. In addition, the diameter of the hole is set to be slightly smaller than the outer diameter (nominal diameter) of the screw.
The fall preventing washer having such a configuration is fitted over the shank of the screw that is inserted through the insertion hole. As the fall preventing washer is fitted over the shank, the tongue-like portions of the fall preventing washer are flexed to enlarge the hole, and the screw is retained in place by the resilient force. Once the screw is assembled to the fall preventing washer and fastened to secure the unit frame to the main-body frame, the screw remains attached to the unit frame even after the unit frame is detached from the main-body frame. This is because, as the unit frame is sandwiched between the screw head and the fall preventing washer. In this way, the screw is prevented from falling out of the unit frame (insertion hole).
Unfortunately, however, the fall preventing washer involves a risk in the case where the axis of the screw is not in precise alignment with the axis of the tapped hole at the time of attaching the fusing unit to the main body of the apparatus. In such a case, the end of the screw may abut against the peripheral wall of the tapped hole and may be fastened in a state where the screw is inclined with the point supported by the fall preventing washer as its center. In that case, the screw is inserted into the tapped hole at an angle, resulting in damaging the threads of the tapped hole and thus making future removable or attachment of the fusing unit difficult. In addition, there is another risk involved in the case where the fusing unit has been removed from the apparatus. If any object accidently hits the unit frame at a location near the screw, which projects from the insertion hole of the unit frame, the screw may be pushed out of the fall preventing washer with great impetus and may fall from the unit frame (fusing unit).
The problems described above are not specific to image forming apparatuses and may arise commonly for any device including a removable unit attached to the main body of the device by fastening screws.